This invention relates to an electrostatic liquid ejecting head with which a solution having charged particles dispersed therein is caused to fly by electrostatic force and deposited on a recording medium to form image. The invention also relates to a process for producing the head.
Various types of ink-jet printing apparatuses have been developed today and they include 1) a thermal type in which ink is heated with a heat-generating resistive element to form a bubble momentarily and the ink is ejected under the pressure of the bubble and 2) a piezoelectric type in which a piezoelectric device is used to convert an electric signal to mechanical vibration that generates a pressure pulse to eject ink. However, the conventional approaches have their own problems; for example, in the thermal type, ink is heated to at least 300° C. and only limited ink materials can be used; in the piezoelectric type, a head structure is necessarily complex which results increasing the equipment cost.
Either type of ink-jet recording apparatus uses ink nozzles and if their size is reduced with a view to increasing the resolution, the solvent evaporates or volatilizes to cause local concentration of the ink, increasing the chance of ink clogging. As a further problem, the conventional nozzle-using ink-jet recording apparatuses are not suitable for improving resolution since ink droplets smaller than 20 micrometer in a diameter are difficult to form.
To form a high-resolution image from an ink-jet recording apparatus, it has been proposed that image be formed by causing ink to fly under electrostatic force. This method requires no nozzles, so one can avoid the problems with ink clogging. What is more, the charged particulate component is condensed at the tip of the head, so ink droplets having very small diameter can be allowed to fly consistently.
However, even in this method, ink will stick to the tip of the head if it is left underrating for a prolonged period. A further problem occurs when ink is ejected continuously; ink supply is not fast enough to prevent the occurrence of fluctuation in the size of ink droplets or considerable drop in the recording frequency.
Techniques that can solve the aforementioned problems have been proposed in JP 9-254372A and JP 10-67114A.
JP 9-254372A relates to an image forming apparatus, such as ink-jet printer, of an electrostatic type. The recording head of the ink-jet printer disclosed in JP 9-254372A and shown in FIG. 14 thereof has a pair of support substrate members laminated each other, each having a plurality of grooves formed on a surface to form ink supply passages and a guide film that is sandwiched between the support substrate members and which has a plurality of projections arranged side by side along the front edge. The individual projections each having a sharp tip are located at openings of the ink supply passages that are formed by the grooves when the pair of support substrate members are laminated together and projections protrude from the openings respectively. A partition wall is provided between adjacent openings such that the wall protrudes from the openings and an ink recovery groove is formed between adjacent partition walls such that the groove extends from the edge of the opening toward the outer peripheral surface of each support substrate member.
According to JP 9-254372A, the carrier fluid separated from a toner particle is pulled up by the capillary action of the ink recovery groove to maintain a stable ink meniscus.
JP 10-67114A relates to an electrostatic printer head, a printer using the head and a method of producing electrodes for use on the head. The method of producing electrodes for use on the printer head as disclosed in JP 10-67114A may be referenced to FIG. 4 thereof. The method comprises the steps of providing a plurality of recesses of different depths in a first substrate by etching, forming an etch stop layer on the surface of the substrate which includes the recesses, forming an electrode layer over the etch stop layer as it fills the recesses over which the etch stop layer has been provided, joining a second substrate in the form of a structural substrate such that the electrode layer is sandwiched between the first substrate and the second substrate, etching away the first substrate, removing the etch stop layer to form bare projecting electrodes, and performing etching to form a groove around each projecting electrode.
According to JP 10-67114A, in order to fabricate the ink-jet printer head, a thermally oxidized insulating SiO2 layer is first formed on the single-crystal Si substrate having the recesses provided by anisotropic etching and the recesses are then filled with a printer head material. Therefore, by controlling the shape of the recesses, one can fabricate a printer head having improved uniformity and reproducing performance.
However, the structure of the recording head on the ink-jet printer disclosed in JP 9-254372A is complex and needs to have a one-dimensional array. Even if two one-dimensional arrays are superposed to form a two-dimensional array, the precision in the positions of the two arrays being superposed will affect the precision of the recording position; this renders the fabrication process difficult to control and increases the fabrication cost.
On the other hand, the recording head fabrication method disclosed in JP 10-67114A can produce a two-dimensional array structure with good precision since it applies the ordinary semiconductor microfabrication technology. However, the method itself is complex enough to increase the manufacturing cost.